Revolutionary War Pensions: Substitutes

When Robert Ashley's father, Jordan Ashley, became ill 2 weeks into a 3 month enlistment in 1779, Robert took his father's place for the duration of the 3 months, then re-enlisted on his own behalf. Robert was age 90 in 1845 when he applied for his Revolutionary War pension! Since he was born about 1755, he was about 24 years old when he began his military service. How old was his father??